FIG. 6 is a perspective view of a semiconductor laser structure disclosed by Fujii et al that employs layers of aluminum gallium indium phosphide (AlGaInP). The laser structure includes an n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P first cladding layer 2, a gallium indium phosphide (Ga.sub.0.5 In.sub.0.5 P) active layer 3, a p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P second cladding layer 4a, and a p-type Ga.sub.0.5 In.sub.0.5 P second cladding layer all successively disposed on and having the same width as an n-type gallium arsenide (GaAs) substrate 1. A stripe-shaped ridge structure 10 including a p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P third cladding layer 4b is disposed on part of the etch stopping layer 90. A p-type GaAs transition layer 5 is part of the ridge structure and is disposed on the third cladding layer 4b. An n-type GaAs current blocking layer 7 is disposed on the etch stopping layer 90 where the ridge structure 10 is not present, i.e., on both sides of and contacting the ridge structure. A p-type GaAs contacting layer 8 is disposed on the ridge structure 10 in contact with the transition layer 5 and on the current blocking layer 7 at both sides of the ridge structure. Electrodes 15 and 16 are disposed on the substrate 1 and the contacting layer 8, respectively. The laser includes opposed facets 17 and 18 at opposite ends of the ridge structure, transverse to the thicknesses of the layers, preferably formed by cleaving.
A method of making the semiconductor laser structure of FIG. 6 is illustrated in FIGS. 7(a)-7(d). Initially, there are successively grown on the n-type GaAs substrate 1 the n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P first cladding layer 2 to a thickness of about one micron, the Ga.sub.0.5 In.sub.0.5 P active layer 3 to a thickness of about 0.1 micron, the p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P second cladding layer 4a to a thickness of about 0.3 micron, the p-type Ga.sub.0.5 In.sub.0.5 P etch stopping layer 90 to a thickness of about four to ten nanometers (nm), the p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P third cladding layer 4b to a thickness of about 0.7 micron, and the p-type GaAs layer 5 to a thickness of about 0.2 micron. Preferably, these layers are grown by metal organic chemical vapor deposition (MOCVD) on a substrate with a (100) surface orientation.
As illustrated in FIG. 7(b), a dielectric film, such as SiN or SiO.sub.2, is deposited on the transition layer 5 and patterned to form a mask 6 over a central portion of the successively deposited layers. A ridge structure having a forward mesa shape, i.e., having a trapezoidal cross-section in the plane of the facets 17 and 18 with sides transverse to the active layer 3 that diverge in the direction of the active layer, is formed by wet etching of the transition layer 5 and the second cladding layer 4a. An etchant that attacks AlGaInP much more rapidly than it attacks GaInP is employed. The etching is effectively controlled, i.e., limited, in the direction of the second cladding layer 4a by the etch stopping layer 90, as shown in FIG. 7(b). Typically, the maximum width w of the ridge structure, i.e., the dimension measured in FIG. 7(b) parallel to the active layer 3 and adjacent the etch stopping layer 90, is three to five microns.
Subsequently, as illustrated in FIG. 7(c), the GaAs current blocking layer 7 is grown, preferably by MOCVD. When MOCVD is used, the current blocking layer 7 does not grow on the mask 6. After removal of the mask 6, in a third and final epitaxial growth step, the contacting layer 8 is grown on the current blocking layer 7 and the transition layer 5. The structure of FIG. 6 is completed by adding the electrodes 15 and 16 and cleaving to form the facets 17 and 18.
When a forward bias voltage is applied across the electrodes 15 and 16, a current flows between the electrodes and is concentrated in a central portion of the active layer 3 by the ridge structure 10, comprising the third cladding layer 4b, and by the current blocking layer 7 and collected through the ridge structure 10 and the contacting layer 8. When a threshold current flow is exceeded, the light generated by recombination of charge carriers in the active layer resonates in the ridge structure between the facets 17 and 18 and laser oscillation occurs. The wavelength of the light produced by the laser depends upon the energy band gap of the material employed in the active layer. When various alloys of GaInP and AlGaAs are used as the active layer material, visible light may be produced.
In addition to the materials just described with respect to the laser structure shown in FIGS. 6 and 7(a)-7(d), other materials can be employed in the laser and result in the generation of visible light. For example, instead of the AlGaInP cladding layers, the first, second, and third cladding layers 2, 4a, and 4b may be Al.sub.0.45 Ga.sub.0.55 As and the active layer 3 may be Al.sub.0.07 Ga.sub.0.93 As. In this embodiment, the etch stopping layer 90 may be present or absent. The thicknesses of corresponding layers and the orientation of the substrate are substantially the same as those previously described with respect the same structure employing different materials. The processing described with respect to FIGS. 7(a)-7(d) is the same as with the other materials including the use of a sulfate etchant, for example, sulfuric acid, for etching a ridge structure that lies along the &lt;011&gt; direction without significantly attacking the etch stopping layer 90, if present. In these structures, referring to FIG. 7(b), the thickness h of the second cladding layer beyond the ridge structure is about 0.2 to 0.3 micron. When the etch stopping layer 90 is absent, the second and third cladding layers 4a and 4b are continuous and are continuously formed in a single growth step. In that case, the etching of the mesa must be more carefully controlled than when the etch stopping layer 90 is present to ensure that the thickness h of the second cladding layer or of a merged second and third cladding layer has the desired value. The operation of the laser employing different materials is identical to that of the initially described laser although the light emission may occur at a different wavelength.
A prior art groove-type or self-aligned semiconductor (SAS) laser structure according to the prior art is shown in FIG. 8. In that figure and in all other figures, the same elements are given the same reference numbers. The principal difference between the SAS laser structure of FIG. 8 and the ridge laser structure of FIG. 6 lies in the current concentration and collection structure. The ridge structure and current blocking layer of FIG. 6 concentrate current in a central part of the active layer 3 for collection by the contacting layer 8. In the laser embodiment of FIG. 8, the third cladding layer 4b does not have the trapezoidal cross-sectional shape of a ridge or mesa but, rather, extends across the entire width of the laser and projects to the second cladding layer 4a in a groove 13 that penetrates through the current blocking layer 7. The respective layers of the lasers of FIGS. 6 and 8 have approximately the same thicknesses. Like the structure of FIG. 6, different materials can be employed in various embodiments of the structure of FIG. 8. For example, the cladding layers may be (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P with an active layer of Ga.sub.0.5 In.sub.0.5 P. Alternatively, the cladding layers may be Al.sub.x Ga.sub.1-x As with an active layer of Al.sub.y Ga.sub.1-y As where x&gt;y. Although the laser structure of FIG. 8 does not include an etch stopping layer 90, such a layer could be included.
FIGS. 9(a)-9(c) illustrate steps in a method of making the laser structure of FIG. 8. Initially, on the substrate 1, the first cladding layer 2, the active layer 3, the second cladding layer 4a, and the current blocking layer 7 are successively grown. As shown in FIG. 9(b), the current blocking layer 7 is etched, preferably with the aid of a mask that is not shown, to form a stripe groove 13 extending through the current blocking layer 7, exposing a central portion of the second cladding layer 4a, and dividing the current blocking layer 7 into two parts.
Subsequently, as illustrated in FIG. 9(c), the third cladding layer 4b and the contacting layer 8 are successively grown on the current blocking layer 7 and on the exposed portion of the second cladding layer 4a between the two parts of the current blocking layer.
To complete the semiconductor laser, the electrodes 15 and 16 are deposited and the facets 17 and 18 are formed by cleaving.
Another prior art SAS semiconductor laser, disclosed in Japanese Published Patent Application 1134985, is shown in a perspective view in FIG. 10. Steps in a method or producing that laser are illustrated in FIGS. 11(a)-11(c). The laser embodiment described in Japanese Published Patent Application 1-134985 has an n-type Al.sub.0.35 Ga.sub.0.65 As cladding layer 2 approximately two microns thick. A buffer layer 1' approximately one micron thick provides a gradual transition between the GaAs substrate 1 and the different material of the cladding layer 2. In this structure, the active layer 3 is GaAs. In place of the third cladding layer 4b having the same composition as the first and second cladding layers 2 and 4a, the laser of FIG. 10 includes a p-type light guide layer 4c of Al.sub.0.25 Ga.sub.0.75 As, a slightly different composition from the cladding layers 2 and 4a. In addition, like the structure of FIG. 6, the structure of FIG. 10 includes a p-type GaInP etch stopping layer 90 about 100 nm thick disposed between the second cladding layer 4a and the current blocking and light guide layers. Otherwise, the layer thicknesses and elements of the laser of FIG. 10 are identical to those already described.
The manufacturing steps shown in FIGS. 11(a)-11(c) are essentially identical to those shown in FIGS. 9(a)-9(c). The difference lies in the presence and exploitation of the GaInP etch stopping layer 90 which controls the depth of the etching that forms the stripe groove 13. In addition, in the laser of FIG. 10, the groove shape is replicated on the surface of contacting layer 8.
An advantage is achieved in laser structures, such as those of FIGS. 6, 8, and 10, in embodiments that employ cladding layers of aluminum gallium arsenide (AlGaAs) when a GaInP etch stopping layer 90 is included in the structure. The aluminum containing layers are spontaneously oxidized when exposed to the atmosphere or in wet etching, for example, in the step illustrated in FIG. 9(b), when a stripe groove or a ridge is formed by etching. When the etch stopping layer 90 is present, it protects the underlying AlGaAs cladding layer from oxygen during etching or atmospheric exposure. When oxidation is prevented, subsequently grown layers, such as the current blocking layer 7 in a ridge structure or the third cladding layer 4b or light guide layer 4c in an SAS structure, are likely to have better crystallinity and electrical characteristics.
Although the etch stopping layer 90 is effective in preventing oxidation of a regrowth interface, its presence can produce problems. For example, a GaInP etch stopping layer is subject to some thermal decomposition, i.e., deterioration, during the growth of subsequent layers. When the etch stopping layer deteriorates, the layers grown on it are of poor quality. In addition, the resistivity of a GaInP etch stopping layer is about two orders of magnitude lower than that of AlGaInP cladding layers when both the cladding layers and the etch stopping layer contain about the same concentration of dopant impurities. Thus, the etch stopping layer can provide a relatively low resistance path on the surface of the adjacent cladding layer. Because of the presence of the lower resistance layer, some of the current flowing through the laser tends to spread beyond the portion of the active layer where there is a gap in the current blocking layer, either in a ridge or groove structure. In other words, the current flowing in the laser is less efficiently concentrated when a GaInP etch stopping layer is present throughout the full width of the structure, as in the prior art lasers.